Fungi cause significant morbidity and mortality. An emerging problem is the susceptibility of patients with leukemia and other cancers to the opportunistic fungi. Despite the prevalence and cost of fungal diseases, the host defense mechanisms active against fungi have not been clearly identified. Recent evidence suggests that transferrin (TF) may mediate host defense against fungi. It is hypothesized that TF mediates host defense by depriving fungi of needed iron, thus inhibiting their growth. We have obtained evidence suggesting that TF's antifungal effect does not result directly from simple iron deprivation but, rather, by events associated with the binding of TF directly to the fungal surface. We hypothesize that TF forms a complex with siderophore (SID) located in the cell wall which leads to the toxic accumulation of SID in the fungal cytoplasm. The mechanism by which TF exerts its antifungal effect will be clarified. The SID-producing, very TF-sensitive Trichophyton mentagropytes and Rhizopus oryzae and the less sensitive Absidia corymbifera, Aspergillus niger, and A. fumigatus will be used. The molecular forms of TF (O-Fe/TF, 1-Fe/TF N- and C-terminal, 2-Fe/TF) that bind to the fungal surface and exert antifungal activity will be identified with urea and gradient gel electrophoresis. The binding of TF to the fungal surface will be detected by immunofluorescence. The association between TF and SID will be extensively investigated with scanning and transmission immunoelectron microscopy. The antifungal effect of bound TF will be evaluated by measuring fungicidal and growth inhibitory activity. Glucose metabolism, RNA synthesis, and ATP formation will be evaluated to determine what component of fungal metabolism is affected by transferrin binding. Participation of iron in the interaction of TF, SID, and fungi will be investigated using 55Fe and 14C-labeled siderphore. The excessive an toxic intracytoplasmic accumulation of SID will be investigated by immunoelectron microscopy and by HPLC analysis of extracted fungal cytosol. The long-term objective is to discover a method of specifically negating transferrin's antifungal activity in vivo so that its role in host defense can be clearly demonstrated. Accomplishment of this objective is dependent upon a clear understanding of how transferrin exerts its antifungal effect.